This is the best answer because the question implies that the blank should be filled in with something that was very common and well known in World War I. To most people, World War I is best known for the trench warfare that was the typical form of combat on the Western Front for most of the war.

However, this type of warfare was not new in the First World War. It had been around at least since the American Civil War. As you can see in the pbs.org link, there was a great deal of trench warfare in the Civil War.

Trench warfare is a good description for the blank in the Civil War. Trenches were part of siege warfare from the beginning of military history. The Civil War used extensive trench systems to connect forts. Richmond, Petersburg, and the Peninsula had extensive earthworks. Newport News, VA has the most extensive Civil War fortifications extant east of the Mississippi River. Trenches were called rifle pits in the Civil War, but they were the same thing. By 1864 both armies had discovered that men shooting from cover fared better than those in the open.

Civil War histories and personal narrative speak of how quickly soldiers dug trenches with bayonets and tin plates. It still took the Union Army a year to break the Confederate earthworks at Petersburg.